ah, i see. that does make a huge difference. i've always thought of rocket tag not as a particular build which can punch the lights out of another build/encounter, but as when the one punched retaliates with something even stronger, and back and forth... hence "tag"; as in the game of tag where kids run around tagging each other. i have always had "rocket tag" defined to me as someone hits someone else, so they hit them back, the first then uses a weapon, so does the second, then the first pulls out a bigger weapon, and it escalates until one or both are using rockets, or similarly overpowered and ridiculous weapons for a supposed game of "tag".
a KO or one-shot result therefore "cannot" be 'tag' based on how i was viewing it and the generally accepted definition of the word tag. thus, in my view, your one-shot-no-defense or save-or-lose or just-lose results... i've never thought of as qualifying for the term "tag". so in summary - tag implies continuing escalation, while one-shotting is a single situation.
now that we have that out of the way: i see that we are actually talking about two different things. you were, if i now understand correctly, speaking all along of a given system's potential for one-shot-little-to-no-defense builds and combos, which certainly is a system specific thing - though i will continue to contend that the mis-use or mis-application of such rules and combos in a fashion inappropriate for a given group's goals remains a human choice, and is thus a human problem with a human solution.
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perhaps i am going so far as to claim that irregardless of the game or rules, mature players won't mis-apply such rules. that is not to say that such rules and combos won't be used, but rather that a mature player will pick and choose carefully where they use something which is overwhelmingly powerful. now having said that... i guess the corollary is that the vast majority of players are therefore... not mature.
and by mature, i don't mean age, or level of system mastery, or even years of experience in gaming. i've met grognards of 20-30+ years experience in gaming who are not mature, and behave in an immature fashion any time they are challenged on something, irrespective of the merits of the challenge.
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also, while i'm meandering about these topics, i would like to contend that the whole glass cannon situation is not necessarily undesirable, and it is deliberately intended to exist in gaming, if not necessarily by the specific rules which yield such powerful combinations. furthermore, i'll go as far as to postulate that game systems which do not provide such options are actually less capable and less well designed.
for an example, i'll take hitting someone over the head. all humans have that weak point, and it is difficult and rare for someone to train sufficient to cover that weakness. as such, even a weak (low level) person can potentially take out a strong (high level) person if the circumstances are right. quite a few historical, literary, and media examples use this exact idea (or a variant of it) as a major plot point. It goes back as far as several thousand years to the babylonian and sumerian cultures, at the least.
despite this well documented fact, most game systems do not have a mechanic which accurately represents this weakness... or rather, in an attempt to (foolishly?) "level the playing field" and break things down into levels, and hit points, and other representational artifacts intended to speed up game play - this simple possibility is overlooked and removed from existence in most games. on the other hand, because of the (poor?) rules design, other loopholes are opened which, when abused by immature gamers of whatever given age/experience/mastery can result in headaches and loss of enjoyment in the course of game play.
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obviously, there are two approaches to solve it - fix the rules, or fix the people. of the two the first would seem easier, but i think that is really just hacking at the leaves and ignoring the roots. and i'm not saying that all games should be super-realistic, nor am i saying that such poor design is excusable because they overlooked something else, somewhere else, like the weakness i pointed out before. then again, trying to fix the people is hard... very hard. and only works if they recognize that there is a problem, and are willing and desire to change in and of themselves....
i get the feeling that putting the blame on the rules/designers, is a form of not taking responsibility on the part of people who misuse and abuse, and a frustrated response on the part of those who are more mature and trying to deal with those who are not.
in any case, when one cannot rely upon the maturity and cooperation of all the players (DM/GM included), that does inherently contain the potential for problems.